Group A streptococci (GAS) cause a wide variety of clinical syndromes, ranging from uncomplicated infections of the pharynx and skin to life-threatening necrotizing fasciitis and streptococcal toxic shock syndrome (Stevens, J Infect Dis 179:S366, 1999). Protection against infection is largely mediated by antibodies against the surface M protein of the organisms. M protein is an alpha-helical, coiled-coil molecule that extends from the surface with its hypervariable amino-terminus exposed to the outside and the conserved carboxy-terminus buried in the cytoplasm. The amino-terminus contains type-specific epitopes that evoke bactericidal antibodies that correlate with protection against the homologous serotype. The emm gene is located in a regulon that is controlled by the upstream positive regulator Mga. Depending on the serotype, the regulon may contain one, two or three emm and emm-like genes. In serotypes containing only one emm gene, deletion or interruption of the emm gene results in an avirulent organism that can no longer resist phagocytosis. In serotypes that express several emm-like genes, each may partially contribute to resistance to phagocytosis, but among the many defined surface proteins of group A streptococci, only antibodies against the M protein have been shown to be opsonic.
GAS are also known to have, or are suspected of having, other virulence determinants, including two cytolytic toxins referred to as streptolysin S (SLS) and streptolysin O (SLO). SLO is a well-characterized, oxygen-labile molecule that lyses eukaryotic cells after binding to membrane cholesterol (Kehoe et al., Infect Immun 55:3228, 1987). SLO is immunogenic in humans and the anti-SLO titer is widely used as an indicator of recent streptococcal infection. Until recently, the characterization of SLS had eluded many investigators. This oxygen-stable toxin is responsible for the β-hemolysis surrounding colonies of GAS grown on blood agar plates (Alouf and Loridan, Methods Enzymol 165:59, 1988). In addition to red blood cells, SLS lyses a wide variety of eukaryotic cells, including myocardial cells, kidney cells, platelets, lymphocytes, and neutrophils (Hryniewicz and Pryjma, Infect Immun 16:730, 1977; Ofek et al., Infect Immun 6:459, 1972). Early studies showed that SLS was an unstable polypeptide with a molecular weight of about 2.8 kDa (Bernheimer, J Bacteriol 93:2024, 1967), which was bound to carrier molecules such as serum albumin, RNA core, or lipoteichoic acid (Theodore and Calandra, Infect Immun 33:326, 1981). On the basis of molecular weight, SLS has been described as the most potent bacterial hemolysin (Wannamaker, Rev Infect Dis 5:S723, 1983). Injection of rabbits with partially purified preparations of SLS resulted in rapid death preceded by intravascular hemolysis and changes in the electrocardiogram (Wannamaker, supra). Unlike SLO, SLS is non-immunogenic, which may be the result of the toxicity of SLS for lymphocytes or possibly because it is always bound to a carrier making potential epitopes cryptic.
Providing polypeptides from streptococci containing non-M protein antigens, especially those that have neutralizing, mucosal, or opsonic epitopes, would enhance therapeutic tools available to protect against a variety of streptococcal infections. Therefore, there is a need in the art for the discovery and characterization of non-M protein antigens that are effective for treating or preventing against such infections, especially antigens that elicit an immune response that is effective against multiple serotypes of group A streptococci. 